I'm currently reading a fascinating book called Pendragon: The Definitive account of the Origins of Arthur, by Steve Blake and Scott Lloyd. They look for the origins of King Arthur and in doing so examine fragments of tales from Welsh histories. The first author to make King Arthur popular was Geoffrey of Monmouth in his Historia Regum Britanniae and he references the early Welsh material, so this is where their investigation begins.
In the early stories though there is no Excalibur. Geoffrey calls Arthur's sword Caliburnus, possibly taken from the Welsh 'Caledfwhlch' meaning Battlebreach or Hard Breach. The name Excalibur was introduced by Chretien de Troyes in 1180, and that is the name I have used in Tom's Inheritance as it is now so closely associated with Arthur.
Excalibur symbolises power and signifies the rightful leader of Britain. It has the power to cut steel and it blinds enemies. It was forged by a skilled blacksmith, probably from the Otherworld or Faery, and was given to Arthur by the Lady of Lake - she was also a late introduction to the tales. In Tom's Inheritance the Lady of the Lake uses Excalibur to bargain with Merlin, for Arthur. To do what? Ah, but you'll have to read the book!
Excalibur's scabbard also had special powers to protect the owner from bleeding to death from battle wounds, however Morgan Le Fay steals it and throws it into a lake, disappearing forever.
Arthur has other weapons, some with magical powers, but I'll talk about those another time. They may make an appearance in my next book, Twice Born.